Structural effects and lipid membrane interactions of the pH-responsive GALA peptide with fatty acid acylation

GALA is a pH-responsive, membrane-perturbing peptide designed to fold from a random coil at physiological pH to an amphipathic α-helix under mildly acidic conditions. Because of its pH-activated function, GALA has been sought-after as a component of intracellular drug delivery systems that could actively propel endosomal escape. In this study, we conjugated GALA with lauryl and palmitoyl fatty acid tails as model hydrophobic moieties and examined the physicochemical characteristics and activities of the resulting peptide amphiphiles (PAs). The fatty acid variants of GALA exhibited distinctly different membrane perturbing mechanisms at pH 7.5 and 5.5. At physiological pH, the PAs ruptured liposomes through a surfactant-like mechanism. At pH 5.5, lauryl-GALA was shown to form transmembrane pores with a higher potency as compared to its unmodified peptide counterpart; however, after prolonged exposure it also caused liposome lysis. The lytic activity of fatty acid-conjugated GALA did not impair cell viability. Lauryl-GALA was tolerated well by SJSA-1 osteocarcinoma cells and enhanced cell internalization of the PA was observed. Our findings are discussed with the overarching goal of developing efficient therapeutic delivery systems.     

pH-Sensitive Liposomes

Abstract

pH sensitive liposomes are lipid compositions that can be destabilized when the external pH is changed; usually from a neutral or slightly alkaline pH to an acidic pH. They are designed to circumvent delivery of liposome contents to the lysosomes of cells following internalization of the vesicle via the endocytic pathway. In the majority of compositions, a lipid containing a pH titratable group is mixed with phosphatidylethanolamine containing unsaturated acyl chains in a molar ratio (pH sensitive component/PE) of 1/4 or greater. There are five major groups of phosphatidylethanolamine containing pH-senstive lipid compositions. These can be classified by their acid-titratable component: phospholipids, acylated amino acids, fatty acids, cholesterol derivatives and miscellaneous double chain amphiphiles. The biophysical mechanism of action involves a transition of the lipids from the lamellar phase to the hexagonal phase. In cell culture, pH sensitive vesicles can increase the delivery of fluorescent markers, proteins, cytotoxic compounds, RNA and DNA into the cytoplasm. The mechanism of delivery is suggested to involve the destabilization of the liposome in the endosome as the pH is reduced from 7.4 to 5.0 and subsequent destabilization of, or fusion with, the endosomal membrane; some of the liposome contents are introduced into the cytoplasm. In most cases, the extent of liposome contents delivery into the cytoplasm is less than 1% of the amount that becomes cell associated. However further studies, with more reliable assays to differentiate cytoplasmic from lysosomal delivery, are required to place an exact value on this efficiency. The efficiency of pH sensitive liposomes in vivo is limited by stability of certain of the liposome compositions in serum and targeting to the appropriate cell. Cholesterol hemisuccinate is a particularly attractive component for in vivo use since it stabilizes the liposome when in serum at pH 7.4. The use of pH sensitive liposomes in drug delivery should continue to expand due to the increasing number of macromolecular therapeutic agents with intracellular targets.     

Whey Protein/Polysaccharide-Stabilized Emulsions: Effect of Polymer Type and pH on Release and Topical Delivery of Salicylic Acid

Emulsions are widely used as topical formulations in the pharmaceutical and cosmetic industries. They are thermodynamically unstable and require emulsifiers for stabilization. Studies have indicated that emulsifiers could affect topical delivery of actives, and this study was therefore designed to investigate the effects of different polymers, applied as emulsifiers, as well as the effects of pH on the release and topical delivery of the active. O/w emulsions were prepared by the layer-by-layer technique, with whey protein forming the first layer around the oil droplets, while either chitosan or carrageenan was subsequently adsorbed to the protein at the interface. Additionally, the emulsions were prepared at three different pH values to introduce different charges to the polymers. The active ingredient, salicylic acid, was incorporated into the oil phase of the emulsions. Physical characterization of the resulting formulations, i.e., droplet size, zeta potential, stability, and turbidity in the water phase, was performed. Release studies were conducted, after which skin absorption studies were performed on the five most stable emulsions, by using Franz type diffusion cells and utilizing human, abdominal skin membranes. It was found that an increase in emulsion droplet charge could negatively affect the release of salicylic acid from these formulations. Contrary, positively charged emulsion droplets were found to enhance dermal and transdermal delivery of salicylic acid from emulsions. It was hypothesized that electrostatic complex formation between the emulsifier and salicylic acid could affect its release, whereas electrostatic interaction between the emulsion droplets and skin could influence dermal/transdermal delivery of the active.     

Preparation of pH-sensitive poly(glycidol) derivatives with varying hydrophobicities: their ability to sensitize stable liposomes to pH

We have previously shown that modification with succinylated poly(glycidol) (SucPG) provides stable egg yolk phosphatidylcholine (EYPC) liposomes with pH-sensitive fusogenic property. Toward production of efficient pH-sensitive liposomes, in this study, we newly prepared three carboxylated poly(glycidol) derivatives with varying hydrophobicities by reacting poly(glycidol) with glutaric anhydride, 3-methylglutaric anhydride, and 1,2-cyclohexanedicarboxylic anhydride, respectively, designated as GluPG, MGluPG, and CHexPG. Correlation between side-chain structures of these polymers and their respective abilities to sensitize stable liposomes to pH was investigated. These polymers are soluble in water at neutral pH but became water-insoluble in weakly acidic conditions. The pH at which the polymer precipitated was higher in the order SucPG < GluPG < MGluPG < CHexPG, which is consistent with the number of carbon atoms of these polymers' side chains. Although CHexPG destabilized EYPC liposomes even at neutral pH, attachment of other polymers provided pH-sensitive properties to the liposomes. The liposomes bearing polymers with higher hydrophobicity exhibited more intense responses, such as content release and membrane fusion, at mildly acidic pH and achieved more efficient cytoplasmic delivery of membrane-impermeable dye molecules. As a result, modification with appropriate hydrophobicity, MGluPG, produced highly potent pH-sensitive liposomes, which might be useful for efficient cytoplasmic delivery of bioactive molecules, such as proteins and genes.     

Calorimetric study on pH-responsive block copolymer grafted lipid bilayers: rational design and development of liposomes

This study is focused on chimeric advanced drug delivery nanosystems and specifically on pH-sensitive liposomes, combining lipids and pH-responsive amphiphilic block copolymers. Chimeric liposomes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and two different forms of block copolymers, i.e. poly(n-butylacrylate)-b-poly(acrylic acid) (PnBA-b-PAA) at 70 and 85% content of PAA at six different molar ratios, each form respectively. PAA block exhibits pH-responsiveness, because of the regulative group of –COOH. –COOH is protonated under acidic pH (pK_a ca. 4.2), while remains ionized under basic or neutral pH, leading to liposomes repulse and eventually stability. Lipid bilayers were prepared composed of DPPC and PnBA-b-PAA. Experiments were carried out using differential scanning calorimetry (DSC) in order to investigate their thermotropic properties. DSC indicated disappearance of pre-transition at all chimeric lipid bilayers and slight thermotropic changes of the main transition temperature. Chimeric liposomes have been prepared and their physicochemical characteristics have been explored by measuring the size, size distribution and ζ-potential, owned to the presence of pH-responsive polymer. At percentages containing medium to high amounts of the polymer, chimeric liposomes were found to retain their size during the stability studies. These results were well correlated with those indicated in the DSC measurements of lipid bilayers incorporating polymers in order to explain their physicochemical behavior. The incorporation of the appropriate amount of these novel pH-responsive block copolymers affects thus the cooperativity, the liposomal stabilization and imparts pH-responsiveness.     

The Three-Dimensional Human Skin Reconstruct Model: a Tool to Study Normal Skin and Melanoma Progression

Most in vitro studies in experimental skin biology have been done in 2-dimensional (2D) monocultures, while accumulating evidence suggests that cells behave differently when they are grown within a 3D extra-cellular matrix and also interact with other cells (1-5). Mouse models have been broadly utilized to study tissue morphogenesis in vivo. However mouse and human skin have significant differences in cellular architecture and physiology, which makes it difficult to extrapolate mouse studies to humans. Since melanocytes in mouse skin are mostly localized in hair follicles, they have distinct biological properties from those of humans, which locate primarily at the basal layer of the epidermis. The recent development of 3D human skin reconstruct models has enabled the field to investigate cell-matrix and cell-cell interactions between different cell types. The reconstructs consist of a "dermis" with fibroblasts embedded in a collagen I matrix, an "epidermis", which is comprised of stratified, differentiated keratinocytes and a functional basement membrane, which separates epidermis from dermis. Collagen provides scaffolding, nutrient delivery, and potential for cell-to-cell interaction. The 3D skin models incorporating melanocytic cells recapitulate natural features of melanocyte homeostasis and melanoma progression in human skin. As in vivo, melanocytes in reconstructed skin are localized at the basement membrane interspersed with basal layer keratinocytes. Melanoma cells exhibit the same characteristics reflecting the original tumor stage (RGP, VGP and metastatic melanoma cells) in vivo. Recently, dermal stem cells have been identified in the human dermis (6). These multi-potent stem cells can migrate to the epidermis and differentiate to melanocytes.     

The potential of pectin as a stabilizer for liposomal drug delivery systems

The aim of the present study was to investigate the potential of different types of pectin as stabilizers for liposomal drug delivery systems. Positively charged liposomes were coated with commercially available and purified low-methoxylated (LM), high-methoxylated (HM) and amidated (AM) pectins. The samples were stored for up to 12 weeks at 4°C, at room temperature and at 35°C. The change in liposomal size and size distribution, zeta potential, pH, leakage of encapsulated carboxyfluorescein (CF), and lipid degradation were studied. All the types of pectin were found to protect the liposomes against aggregation during storage. The pectin coat did not affect the permeability of the liposome membrane. HM and LM pectin seemed to be the most promising types of pectin due to minimal changes in the zeta potentials during storage for these samples and no detectable lipid degradation. It is concluded that pectin may be used for stabilizing liposomal drug delivery systems.     

Cytosolic delivery of macromolecules. II. Mechanistic studies with pH-sensitive morpholine lipids

Drug carriers containing weak acids or bases can promote cytosolic delivery of macromolecules by exploiting the acidic pH of the endosome. We have prepared two pH-sensitive mono-stearoyl derivatives of morpholine, one with a (2-hydroxy) propylene (ML1) linker and the other, an ethylene (ML2) linker. The pK(a) values of lipids ML1 and ML2, when incorporated into liposomes, are 6.12 and 5.91, respectively. Both lipids disrupt human erythrocytes at pH equal to or below their pK(a) but show no such activity at pH 7.4. Confocal microscopy studies suggest partial endosome-to-cytosol transfer of fluorescent dextran (MW 10 kDa) encapsulated in liposomes that contained 20 mol% of morpholine lipids. Interestingly, co-incubation of morpholine lipids in free or micellar form (without liposomal incorporation) with dextran resulted in efficient cytosolic delivery. Upon acidification to the endosomal pH, liposomes containing ML1 revealed: (a). leakage of entrapped solute that is independent of solute size; (b). lack of liposomal collapse into micelles as evidenced by photon correlation spectroscopy and UV light scattering; and (c). minimal inter-bilayer interactions as shown in a fluorescence resonance energy transfer assay. These observations are consistent with progressive intravesicular reorganization of lipids into stable liposomes of smaller size, but of more homogeneous distribution, upon acidification. The results emphasize a need to manipulate liposomal formulations containing ML1 such that ML1 will promote catastrophic collapse of liposomes to mixed micelles upon exposure to acidic pH. It is only then that micelle-mediated permeabilization of the endosomal membrane will lead to efficient cytosolic delivery of macromolecules originally loaded in liposomes.     

In vitro evaluation of archaeosome vehicles for transdermal vaccine delivery

Abstract

Archaeosomes composed of archaeal total polar lipids (TPL) or semi-synthetic analog vesicles have been used as vaccine adjuvants and delivery systems in animal models for many years. Typically administered by intramuscular or subcutaneous injections, archaeosomes can induce robust, long-lasting humoral and cell-mediated immune responses against entrapped antigens and provide protection in murine models of infectious disease and cancer. Herein, we evaluated various archaeosomes for transdermal delivery, since this route may help eliminate needle-stick injuries and needle re-use, and therefore increase patient compliance. Archaeosomes composed of TPL from different archaea (Halobacterium salinarum, Methanobrevibacter smithii, Haloferax volcanii) and various semi-synthetic glycolipid combinations were evaluated for their ability to diffuse across the skin barrier using an ex vivo pig skin model and the results were compared to conventional synthetic ester liposomes. Physicochemical characteristics were determined for selected formulations including vesicle size, size distribution, zeta potential, fluidity, antigen (ovalbumin) incorporation efficiency and release. Archaeosomes, in particular those composed of M. smithii TPL or the synthetic glycolipid sulfated S-lactosylarchaeol (SLA) mixed with uncharged glycolipid lactosyl archaeol (LA), appeared to be effective carriers for ovalbumin, achieving much better antigen distribution and vesicle accumulation in the skin epidermis than conventional liposomes. The enhanced skin permeation of archaeosomes may be attributed to their chemical structure and physicochemical properties such as particle size, surface charge, stability, and fluidity of their lipid bilayer.     

Role of AUF1 and HuR in the pH-responsive stabilization of phosphoenolpyruvate carboxykinase mRNA in LLC-PK₁-F⁺ cells

Onset of metabolic acidosis leads to a rapid and pronounced increase in expression of phosphoenolpyruvate carboxykinase (PEPCK) in rat renal proximal convoluted tubules. This adaptive response is modeled by treating a clonal line of porcine LLC-PK(1)-F(+) cells with an acidic medium (pH 6.9, 9 mM HCO(3)(-)). Measurement of the half-lives of PEPCK mRNA in cells treated with normal (pH 7.4, 26 mM HCO(3)(-)) and acidic medium established that the observed increase is due in part to stabilization of the PEPCK mRNA. The pH-responsive stabilization was reproduced in a Tet-responsive chimeric reporter mRNA containing the 3'-UTR of PEPCK mRNA. This response was lost by mutation of a highly conserved AU sequence that binds AUF1 and is the primary element that mediates the rapid turnover of PEPCK mRNA. However, siRNA knockdown of AUF1 had little effect on the basal levels and the pH-responsive increases in PEPCK mRNA and protein. Electrophoretic mobility shift assays established that purified recombinant HuR, another AU element binding protein, also binds with high affinity and specificity to multiple sites within the final 92-nucleotides of the 3'-UTR of the PEPCK mRNA, including the highly conserved AU-rich element. siRNA knockdown of HuR caused pronounced decreases in basal expression and the pH-responsive increases in PEPCK mRNA and protein. Therefore, basal expression and the pH-responsive stabilization of PEPCK mRNA in LLC-PK(1)-F(+) cells, and possibly in the renal proximal tubule, may require the remodeling of HuR and AUF1 binding to the elements that mediate the rapid turnover of PEPCK mRNA.     

Temperature- and ionic strength-induced conformational changes in the lipid head group region of liposomes as suggested by zeta potential data.

Neutral liposomes composed of DMPC (dimyristoylphosphatidylcholine), DPPC (dipalmitoylphosphatidylcholine) or DSPC (distearoylphosphatidylcholine) are found to exhibit non-zero zeta potentials in an electric field even when they are dispersed in solution at pH 7.4. A model for the orientation of lipid head groups is proposed to explain the observed non-zero zeta potentials. The dependence of the zeta potential on temperature and ionic strength is analyzed via this model to obtain the information on the direction of the lipid head group in the liposome surface region. The direction of the lipid head group is found to be sensitive to the temperature and the ionic strength of the medium. At low ionic strengths, the phosphatidyl groups are located at the outer portion of the head group region. At constant temperature, as the ionic strength increases, the choline group approaches the outer region of the bilayer surface while the phosphatidyl group hides behind the surface. At the phase transition temperature of the lipid, the phosphatidyl group lies in the outer-most region of the surface and the choline group is in the inner-most region.     

Design and development of ethosomal transdermal drug delivery system of valsartan with preclinical assessment in Wistar albino rats

Abstract

Valsartan (VLT) is a highly selective and orally active antihypertensive drug. However, its oral administration is associated with drawbacks like low bioavailability. The objective of this study was to design and develop a transdermal delivery system for VLT using ethosomal carriers to investigate their enhanced transdermal delivery potential. VLT ethosomes were prepared by cold method. VLT ethosomes were characterized by scanning electron microscopy. The prepared ethanolic liposomes were characterized to be spherical having low polydispersity of nano-size range with good entrapment efficiency. ETC5 ethosomal suspension with 4% of phospholipon 90H and 40% of ethanol was found to have highest entrapment efficiency, i.e. 80.230?±?0.8748%. The permeation study of ethosomes was evaluated by ex vivo diffusion study through rat abdominal skin using Franz’s diffusion cells and ETC5 ethosomal suspension was found to have highest permeation with flux of 92.819?±?1.539?µg/cm²/h, when compared to the permeation profiles of drug solutions either in water or in a water–ethanol mixture. Transdermal application of ethosomal VLT on Wistar rats showed better and prolonged antihypertensive activity in comparison to orally administered VLT suspension by virtue of transdermal permeation through Wistar rat skin. Histopathological study of skin applied with ETC5 showed intercellular permeation across skin by dissolving intercellular lipids in epidermis without causing any rigorous changes in the skin cellular structure. In conclusion, ethosomes enabled the transdermal permeation of VLT, which amply proves its superiority over oral administration for antihypertensive treatment.     

Interaction of Na2B12H11SH with dimyristoyl phosphatidylcholine liposomes

Previous investigations have revealed that the boron cluster compound Na₂B₁₂H₁₁SH (BSH) is very potent in causing major structural rearrangements of and leakage from phosphatidylcholine liposomes. This somewhat unexpected finding is interesting from a fundamental point of view and may also constitute the basis of future important pharmaceutical/medical applications of BSH. In order to further explore the BSH-lipid interaction, we have studied the effects caused by BSH on dimyristoyl phosphatidylcholine (DMPC) liposomes.Cryo-transmission electron microscopy showed that BSH induces aggregation, membrane rupture and increasing wall thickness of the liposomes. Differential scanning calorimetry revealed a BSH dependent shift of the gel to liquid crystalline phase transition temperature of DMPC. The zeta potential of the liposomes decreases with increasing BSH concentrations, and an apparent dissociation constant of 0.23 mM was found.BSH caused leakage of liposome-encapsulated carboxyfluorescein; leakage was higher at 23 °C (near the phase transition temperature) than at 15 °C and 37 °C. It induced lipid mixing only at very high concentrations.     

Nanoemulsion based gel for transdermal delivery of meloxicam: Physico-chemical,mechanistic investigation

AimsThe aim of the present investigation was to develop a nanoemulsion (NE) gel formulation for the transdermal delivery of meloxicam (MLX) in order to ensure maximum controlled and sustained drug release capacity.Main methodsThe MLX containing NE gel was prepared and characterized for particle size, zeta potential, pH, rheology, in vitro drug release, in vitro skin permeation, and in vitro hemolysis. Differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) of MLX-NE gel treated rat skin was performed to investigate the skin permeation mechanism of meloxicam from NE gel. Skin permeation potential of the developed gel formulation was assessed using confocal laser scanning microscopy (CLSM). The in vivo toxicity of MLX-NE gel was assessed by histopathological examination in rat. The rat paw edema test was performed to evaluate the anti-inflammatory activity of MLX-NE gel.Key findingsPercutaneous absorption studies demonstrated a higher permeation of meloxicam from NE gel, than the drug solution. FTIR and DSC studies supported stratum corneum lipid extraction as a possible penetration enhancer mechanism for MLX-NE gel. CLSM studies confirmed the permeation of the NE gel formulation to the deeper layers of the skin (up to 130 μm). MLX-NE gel turned out to be non-irritant, biocompatible, and provided maximum inhibition of paw edema in rats over 24 h in contrast to MLX solution.SignificanceThe nanoemulsion gel formulation may hold promise as an effective alternative for the transdermal delivery of meloxicam.     

Dual signal-responsive liposomes for temperature-controlled cytoplasmic delivery

For production of a new type of functional liposome whose destabilization can be triggered by a combination of a temperature signal and acidic pH signal, we prepared liposomes modified with hyperbranched poly(glycidol) derivatives having N-isopropylamide and carboxyl groups. HeLa cells incubated with the dual signal-responsive liposomes encapsulating a water-soluble fluorescent dye pyranine at 28 °C displayed punctate fluorescence of pyranine, indicating that the liposomes were trapped in endosome. However, after heating at 45 °C for 15 min, the same cells exhibited diffuse fluorescence of pyranine, indicating that destabilization of the liposomes in endosome with an acidic environment in combination with the brief heating caused efficient transfer of the contents into cytosol. The dual signal-responsive liposomes might have usefulness for site-specific delivery of membrane-impermeable molecules, which exhibit bioactivities in the intracellular spaces, such as siRNA and proteins.     

Permeation of skin with (C60) fullerene dispersions

Dispersions in transcutol/isopropyl myristate make C₆₀ fullerene molecules suitable for transdermal delivery. We found that C₆₀ can successfully permeate the skin using pig skin in Franz diffusion cells. Molecular dynamics simulations and transmission electron microscopy confirmed these observations. Basic cosmetic formulations with transcutol/isopropyl myristate without harsh organic solvents show a high potential for delivery of C₆₀ for biopharmaceutical and cosmetics applications.     

Interaction of nanoparticles and cell-penetrating peptides with skin for transdermal drug delivery

Abstract

Topical or transdermal drug delivery is challenging because the skin acts as a natural and protective barrier. Therefore, several methods have been examined to increase the permeation of therapeutic molecules into and through the skin. One approach is to use the nanoparticulate delivery system. Starting with liposomes and other vesicular systems, several other types of nanosized drug carriers have been developed such as solid lipid nanoparticles, nanostructured lipid carriers, polymer-based nanoparticles and magnetic nanoparticles for dermatological applications. This review article discusses how different particulate systems can interact and penetrate into the skin barrier. In this review, the effectiveness of nanoparticles, as well as possible mode of actions of nanoparticles, is presented. In addition to nanoparticles, cell-penetrating peptide (CPP)-mediated drug delivery into the skin and the possible mechanism of CPP-derived delivery into the skin is discussed. Lastly, the effectiveness and possible mechanism of CPP-modified nanocarriers into the skin are addressed.     

Amyloid Assemblies of Influenza A Virus PB1-F2 Protein Damage Membrane and Induce Cytotoxicity

PB1-F2 is a small accessory protein encoded by an alternative open reading frame in PB1 segments of most influenza A virus. PB1-F2 is involved in virulence by inducing mitochondria-mediated immune cells apoptosis, increasing inflammation, and enhancing predisposition to secondary bacterial infections. Using biophysical approaches we characterized membrane disruptive activity of the full-length PB1-F2 (90 amino acids), its N-terminal domain (52 amino acids), expressed by currently circulating H1N1 viruses, and its C-terminal domain (38 amino acids). Both full-length and N-terminal domain of PB1-F2 are soluble at pH values ≤6, whereas the C-terminal fragment was found soluble only at pH ≤ 3. All three peptides are intrinsically disordered. At pH ≥ 7, the C-terminal part of PB1-F2 spontaneously switches to amyloid oligomers, whereas full-length and the N-terminal domain of PB1-F2 aggregate to amorphous structures. When incubated with anionic liposomes at pH 5, full-length and the C-terminal part of PB1-F2 assemble into amyloid structures and disrupt membrane at nanomolar concentrations. PB1-F2 and its C-terminal exhibit no significant antimicrobial activity. When added in the culture medium of mammalian cells, PB1-F2 amorphous aggregates show no cytotoxicity, whereas PB1-F2 pre-assembled into amyloid oligomers or fragmented nanoscaled fibrils was highly cytotoxic. Furthermore, the formation of PB1-F2 amyloid oligomers in infected cells was directly reflected by membrane disruption and cell death as observed in U937 and A549 cells. Altogether our results demonstrate that membrane-lytic activity of PB1-F2 is closely linked to supramolecular organization of the protein.     

Solubilization of Multilamellar Liposomes in the Presence of Non-ionized Drug

The solubilization of multilamellar liposomes by metoprolol tartrate (MPL) has been studied as a function of pH, [MPL], [dimyristoylphosphatidylcholine (DMPC)], temperature and lipid composition. The solubilization of liposomes at 37° C by 7.3 mM MPL occurred at different rates at different pH values. MPL completely solubilized by 7.2 mM DMPC liposomes after about 17 hat pH 12, but only a partial solubilization occurred at pH 10 and 11. Between pH 7 and 9 no change in turbidity was observed after 1 week. Addition of cholesterol (CHOL) to DMPC (2:1 mol) had very little effect on solubilization after 24 h, however with DMPC:CHOL (5:1 mol) the decrease in turbidity was observed after 24 h, even though solubilization was much less compared with that of DMPC alone. The rate of solubilizaiton was decreased when dipalmitoylphosphatidylcholine liposomes were employed. Addition of dicetylphosphate (DCP) to DMPC liposomes reduced the rate of solubilization significantly. The solubilization of liposomes by 7.3 mM MPL as a function of [DMPC], indicated that the lower the liposome concentration the greater the effect on solubilization. It is concluded that MPL in the non-ionized form has a solubilizing effect on liposomes, and addition of CHOL or DCP to DMPC has a stabilizing effect against solubilization.     

Fluidity enhancement: a critical factor for performance of liposomal transdermal drug delivery system

Liposomes are well known lipid carriers for drug delivery of bioactive molecules encapsulated inside their membrane. Liposomes as skin drug delivery systems were initially promoted primarily for localized effects with minimal systemic delivery. Subsequently, a novel vesicular system, transferosomes was reported for transdermal delivery with efficiency similar to subcutaneous injection. The multiple bilayered organizations of lipids applied in these vesicles structure are somewhat similar to complex nature of stratum corneal intercellular lipids domains. The incorporation of novel agents into these lipid vesicles results in the loss of entrapped markers but it is similar to fluidization of stratum corneum lipids on treatment with a penetration enhancer. This approach generated the utility of penetration enhancers/fluidizing agents in lipids vesicular systems for skin delivery. For the transdermal and topical applications of liposomes, fluidity of bilayer lipid membrane is rate limiting which governs the permeation. This article critically reviews the relevance of using different types of vesicles as a model for skin in permeation enhancement studies. This study has also been designed to encompass all enhancement measurements and analytical tools for characterization of permeability in liposomal vesicular system.